plus business demand for investment, might not be sufficient to maintain a high level of employment. business investment demand collapsed in the depression. nobody was building new plants. they were already operating with unused capacity. keynes showed that this fall and investment demand might bring down consumers' demand as well. "what determines consumption demand?" keynes asked. basically, the level of income in the economy. if my income is $10,000 a year, i'll spend, say, $9,000 on consumers' goods. if my income is $20,000, i'll spend, say, $17,000. if my income is only $4,000, i'll want to spend only $3,900 on consumption. we can see what happens in the keynesian world when investment falls. this fall brings down income here.